The present invention is related to methods for preventing volatile organic compounds (VOC) emissions from storage vessels and maintaining acceptable pressure within such vessels.
Volatile organic compounds (VOCS) are generally stored at ambient pressure and temperature in low pressure storage tanks. The VOCs coexist in the liquid and vapor phases in the tanks. The tanks experience depressurization/pressurization due to filling and draining of the tanks as well as due to changes in external temperature and, to a lesser degree, changes in pressure. To avoid tank rupture due to pressure increases within the tanks, it is necessary to vent vapors when the pressure rises. Since these vapors include VOCs, they typically cannot be vented to the atmosphere due to environmental concerns. To avoid tank collapse due to pressure decreases within the tanks, it is necessary to replenish vapors when the pressure falls. This is done by adding an inert or uncondensible gas such as nitrogen or air as a blanket gas. The blanket gas is also sometimes called a pad gas.
Various methods have been developed to remove VOCs prior to the remainder of the vapor containing the blanket gas being vented to the atmosphere. The principle technologies commercially available for reducing VOC emissions are:
(1) granular activated carbon adsorption plants; PA0 (2) carbon cloth adsorption plants; PA0 (3) molecular sieve adsorbents; PA0 (4) thermal oxidation/incineration; PA0 (5) catalytic decomposition; and PA0 (6) compression/refrigeration systems. PA0 providing a single adsorber with a selectively openable and closable vent to the atmosphere (adsorber vent) and filled with an adsorbent that adsorbs VOCs and from which said VOCs are desorbed by applying vacuum and purging with inert gas; PA0 providing a selectively openable and closable first line between the tank and the adsorber; PA0 opening the first line in response to a pressure rise within the tank thereby directing said vapor containing the VOCs to the adsorber wherein the VOCs are adsorbed; PA0 opening the adsorber vent proportionally to a pressure rise within the storage tank thereby directing vapor essentially free of VOCs to the atmosphere; PA0 providing a source of selectively applicable vacuum; PA0 providing a source of selectively applicable inert gas; PA0 providing a selectively openable and closable second line between the inert gas source and the adsorber; PA0 providing a selectively openable and closable third line between the vacuum source and the adsorber; PA0 providing a selectively openable and closable fourth line between the vacuum source and the storage vessel's vapor space; PA0 opening the second, third, and fourth lines in response to a pressure drop within the storage vessel below a predetermined pressure thereby directing the inert gas through the adsorber at a vacuum thus desorbing the VOCs, said VOCs and inert gas being directed to the storage tank's vapor space through the third and fourth lines; and PA0 providing a means for the automatic control and monitoring of the system comprising means to start and stop the vacuum source, to open and close the lines, and to monitor the pressure, temperature and flow.
These technologies either destroy the VOCs, generating a waste that must be disposed of, or recover the VOCs in liquid form that must be handled to be recycled. There remains a need for a process for controlling vapor phase emissions of VOCs that is efficient and economical.